1. Field of the Invention
The present invention relates to a turbo-molecular pump for evacuating gas by using a high speed rotor.
2. Description of the Related Art
An example of a conventional turbo-molecular pump is shown in FIG. 37. The turbo-molecular pump is comprised by a cylindrical pump casing 14 housing a vane pumping section L1 and a groove pumping section L2 which are constituted by a rotor (rotation member) R and a stator (stationary member) S. The bottom portion of pump casing 14 is covered by a base section 15 which is provided with an exhaust port 15a. The top portion of pump casing 14 is provided with a flange section 14a for coupling the pump to an apparatus or a piping to be evacuated. Stator S comprises a stator cylinder section 16 provided upright at the center of the base section 15, fixed sections of vane pumping section L1 and groove pumping section L2.
Rotor R is comprised by a rotor cylinder section 12 attached to a main shaft 10 which is inserted into stator cylinder section 16. Between main shaft 10 and stator cylinder section 16 are constituted a drive motor 18, an upper radial bearing 20 and a lower radial bearing 22 disposed on the upper and lower sides of drive motor 18 respectively. Under main shaft 10, there is an axial bearing 24 having a target disk 24a at the bottom end of main shaft 10 and an upper and a lower electromagnets 24b on the stator S side. In this configuration, a high speed rotation of rotor R is supported under a five coordinate active control system.
Rotor vanes 30 are provided integrally with the upper external surface of rotor cylinder section 12 to form an impeller, and on the inside of pump casing 14, stator vanes 32 are provided in such a way to alternately interweave with rotor vanes 30. These vane members constitute vane pumping section L1 which carries out gas evacuation by cooperative action of the high speed rotor vanes 30 and stationary stator vanes 32. Below vane pumping section L1, groove pumping section L2 is provided. Groove pumping section L2 is comprised by a spiral groove section 34 having spiral grooves 34a on the outer surface of the bottom end of rotor cylinder section 12, and stator S comprises a spiral groove section spacer 36 surrounding spiral groove section 34. Gas evacuation action of groove pumping section L2 is due to the dragging effect of spiral grooves 34a of spiral groove section against gases.
By providing groove pumping section L2 at downstream of vane pumping section L1, a wide-range turbo-molecular pump can be constructed so as to enable evacuation over a wide range of gas flow rates using one pumping unit. In this example, the spiral grooves of groove pumping section L2 are provided on the rotor side of the pump structure, but some pumps have the spiral grooves formed on the stator side of the pump structure.
Such turbo-molecular pumps are assembled as follows. Firstly, groove pumping section spacer 36 is attached by coupling the lower surface of step 36a to protruded ring section 15b formed on base section 15. Next, rotor R is fixed in some position, and stator vanes 32, which are normally split into two half sections, are clamped around to interweave between rotor vanes 30. This is followed by placing a stator vane spacer 38, having steps on its top and bottom regions, on top of the clamped rotor vane 30. This assembling step is repeated for each rotor vane 30 to complete the assembly of stator vanes 32 around rotor R.
Lastly, pump casing 14 is attached by sliding it around the layered stator vane structure and fixing flange 14b to the base 15 of stator S by fasteners such as bolts, thereby pressing the top stator vane spacer 38 firmly against stepped surface 14c on the inside surface of pump casing 14 and binding the entire layered assembly and groove pumping section spacer 36. It can be understood from this assembly structure that the peripheries of each of stator vanes 32 are pressed together by stator vane spacers 38 located above and below, and similarly groove pumping section spacer 36 is pressed down by the lowermost stator vane 32, stator vane spacer 38 and protrusion section 15b of base section 15, so that the axially applied pressing force prevents induced rotation of stator vanes 32 and groove pumping section spacer 36 with rotor R in the circumferential direction.
Also, though not shown in the drawing, in order to ensure to fix the groove pumping section spacer 36 to the stator cylinder section 16 of the stator S, sometimes groove pumping section spacer 36 is fastened to stator cylinder section 16 of stator S by bolts to assure such fixation.